Field of the Invention
The present invention relates to radio communications apparatus and methods of communicating data using radio signals. More specifically, the present invention relates to receivers that recover data for at least one of a plurality of users from spread spectrum radio signals. Furthermore, the present invention relates to a method of recovering data for at least one of a plurality of users from spread spectrum radio signals.
Data is communicated using code division multiple access systems (CDMA) by combining the data with a user specific spreading code and transmitting the combination to a receiver using radio signals. At the receiver, the data is recovered from the radio signals by comparing the received sampled radio signals with a user specific spreading code that is known to the receiver. Similarly, a plurality of transmitters operatively coupled to a corresponding plurality of user data sources can be configured to contemporaneously communicate data from the transmitters to the receivers which detect the data by comparing the radio signals with the user specific spreading code uniquely associated with each of the users.
A result of combining the data with a user specific spreading code is that the bandwidth obtained from the resulting radio signals is greater than that obtained by radio signals carrying data alone. Accordingly, the frequency bandwidth occupied by code division multiple access signals is often greater than a coherence bandwidth of the radio communications channel through which the signals are transmitted. Code division multiple access communicated radio signals provide an advantage because the signals themselves exhibit frequency diversity, and different parts of the bandwidth of the radio signals suffer from fading independently.
Code division multiple access is used in second generation mobile radio telephone systems, and is now being proposed for third generation mobile radio telephone systems. A characteristic of a frequency bandwidth in which radio signals are transmitted in second and third generation mobile radio telephone systems is that the radio signals travel between the transmitters and the receivers of the system through a plurality of paths. As a result, data symbols regenerated from the received signals exhibit inter symbol interference that must be cancelled in order for the data to be recovered.
A further advantage of code division multiple access is that radio signals traveling through each of the paths that reach the receiver may be individually resolved and the radio signal energy for each of these signals can be recovered and combined to facilitate detection of the communicated data. A receiver that operates in this way to detect radio signals traveling through individual paths is known as a rake receiver. A rake receiver is provided with a plurality of rake fingers. Each rake finger is configured to correlate the received signals with the user specific spreading code. Each one of the fingers of the rake receiver is assigned to one of a plurality of temporal displacements corresponding to one of a plurality of paths through which the radio signals reach the receiver.
A further known receiver for CDMA spread spectrum radio signals is described in a published article entitled xe2x80x9cBlind Adaptive 2D Rake Receiver for DS-CDMA Based on Space-Frequency MVDR Processing,xe2x80x9d by Zoltowski et al., which is known to have been submitted for publication to the IEEE Journal on Transactions on Signal Processing, June 1996, and is currently available on the Internet. This article discloses a rake receiver operatively associated with an array of antennas that are configured to provide spatial diversity and antenna gain with respect to the detected radio signals. Furthermore, each of the rake fingers operates to correlate the received signals and to form a frequency domain representation of the correlated signals to detect and recover data from the detected radio signals. This rake receiver is referred to in the following description as a two dimensional space-frequency rake receiver.
Mobile radio telecommunications systems are configured to use an allocated portion of the radiofrequency spectrum as efficiently as possible. The ability of a receiver of CDMA radio signals to recover data in the presence of contemporaneously detected radio signals substantially contributes to the efficiency of CDMA communication systems.
It is accordingly an object of the invention to provide an improved receiver for detecting and recovering data communicated using code division multiple access, and an improved method of detecting and recovering data communicated using code division multiple access, which overcome the above-mentioned disadvantageous of the heretofore known receivers and methods of this type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a receiver for recovering data, for at least one of a plurality of users, from spread spectrum radio signals that include radio signals representative of a combination of data and a data spreading code associated with the at least one of the plurality of users and a combination of a predetermined sequence of pilot data and a pilot spreading code. The improvement includes a plurality of antennas, with each one of the antennas configured to detect a different version of the radio signals. A base band conversion device is electrically coupled to the antennas and configured to generate, for each version of the radio signals, a version of baseband signal samples representative of the respective version of the radio signals. A data recovery device is configured to: correlate each version of the baseband signal samples with respect to the pilot spreading code to obtain respective versions of correlated pilot spreading code signal samples; form at least one covariance matrix from a frequency domain representation of a predetermined temporal window of the respective versions of the correlated pilot spreading code signal samples; correlate each version of the baseband signal samples with respect to the data spreading code to obtain respective versions of correlated data spreading code signal samples; form a frequency domain representation of each version of the correlated data spreading code signal samples; and detect the data from the frequency domain representation of each version of the correlated data spreading code signal samples in combination with the at least one covariance matrix.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a receiver for recovering data, for at least one of a plurality of users, from spread spectrum radio signals that include radio signals representative of a combination of data and a data spreading code associated with the at least one of the plurality of users and a combination of a predetermined sequence of pilot data and a pilot spreading code. The improvement includes a plurality of antennas. Each one of the antennas is configured to detect a different version of the radio signals. A base band conversion device is electrically coupled to the antennas and configured to generate, for each version of the radio signals, a version of baseband signal samples representative of the respective version of the radio signals. A data recovery device includes a plurality of rake detectors. Each one of the rake detectors is electrically coupled to the base band conversion device to obtain a respective version of the base band signal samples. Each one of the rake detectors has: a correlation device to correlate the respective version of the base band signal samples with the data spreading code to obtain correlated data spreading code signal samples and to correlate the respective version of the base band signal samples with the pilot spreading code to obtain correlated pilot spreading code signal samples, and a frequency conversion device electrically coupled to the correlation device to generate a frequency domain representation of the correlated data spreading code signal samples and of the correlated pilot spreading code signal samples. The data recovery device also includes: a combiner device electrically coupled to each one of the rake detectors to form at least one covariance matrix from a frequency domain representation of a predetermined temporal window of the correlated pilot spreading code signal samples obtained from each one of the rake detectors and to combine the covariance matrix with the frequency domain representation of the correlated data spreading code signal samples and of the correlated pilot spreading code signal samples obtained from each one of the rake detectors; and a data detection device to detect data symbols from the combination of the covariance matrix and the frequency domain representations.
In accordance with an added feature of the invention, the covariance matrix formed by the combiner device includes a signal plus interference and noise space-frequency covariance matrix obtained from the frequency domain representation of the predetermined temporal window of the correlated pilot spreading code signal samples obtained from each one of the rake detectors. The temporal window is taken in temporal correspondence with the predetermined sequence of pilot data.
In accordance with an additional feature of the invention, the covariance matrix formed by the combiner device includes an interference and noise-covariance matrix obtained from the predetermined temporal window of the correlated pilot spreading code signal samples obtained from each one of the rake detectors. The temporal window is taken at different temporal displacements. Note that the interference and noise covariance matrix corresponding to one slot can either be estimated by applying the temporal window to the output of the correlator with the pilot sequence in which case the temporal window may not comprise any multipath components. Or the interference and noise covariance matrix can be estimated by applying the temporal window to the signal before it is passed through the correlator.
In accordance with another feature of the invention, the frequency domain representation of the correlated data spreading code signal samples and of the correlated pilot spreading code data signal samples generated by each one of the rake detectors are combined by the combiner device to form a data signal plus interference and noise space-frequency snapshot.
In accordance with a further feature of the invention, the combiner device estimates a weight vector from a matrix selected from the group consisting of a signal space-frequency covariance matrix and a signal plus interference and noise space-frequency co-variance matrix, in combination with an interference and noise-covariance matrix; and multiplies an Hermitian transpose of the weight vector by the data signal plus interference and noise space-frequency snapshot. The data detection device, detects the data symbols from the result of the multiplication.
In accordance with a further added feature of the invention, the predetermined temporal window is taken substantially in accordance with a multiple path delay spread experienced by the radio signals.
In accordance with a further additional feature of the invention, the correlator device includes a first and a second correlator. The first correlator is configured to correlate the respective version of the base band signal samples with the pilot spreading code to obtain the correlated pilot spreading code signal samples. The second correlator can be configured to correlate the respective version of the base band signal samples with the data spreading code to obtain the correlated data spreading code signal samples.
In accordance with yet another feature of the invention, the frequency conversion device is a discrete Fourier transformer or e.g. a Fast transformer.
With the foregoing and other objects in view there is also provided, in accordance with the invention, an apparatus for communicating data with radio signals, including a device for generating spread spectrum radio signals representative of a combination of data and a data spreading code associated with at least one user, and a combination of a predetermined sequence of pilot data and a pilot spreading code. A plurality of antennas is provided, with each one of the plurality of antennas configured to detect a different version of the radio signals. A base band conversion device is coupled to the plurality of antennas and configured to generate, for each antenna, a version of base band signal samples representative of a respective version of the radio signals. A data recovery device is included for: correlating each version of the base band signal samples with respect to the pilot spreading code to obtain correlated pilot spreading code signal samples; forming at least one covariance matrix from a frequency domain representation of a predetermined temporal window of the correlated pilot spreading code signal samples; correlating each version of the base band signal samples with respect to the data spreading code associated with the user to obtain correlated data spreading code signal samples; forming a frequency domain representation of each version of the correlated data spreading code signal samples; and detecting the data from the frequency domain representation of each version of the correlated data spreading code signal samples in combination with the at least one covariance matrix.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method of recovering data for at least one of a plurality of users from spread spectrum radio signals that include radio signals generated from a combination of the data and a data spreading code associated with the at least one of the plurality of users, and radio signals generated from a combination of a predetermined sequence of pilot data and a pilot spreading code. Spread spectrum radio signals are detected at each one of a plurality of antennas. The spread spectrum radio signals include radio signals generated from a combination of data and a data spreading code associated with a user, and radio signals generated from a combination of a predetermined sequence of pilot data and a pilot spreading code. For each one of the detected spread spectrum radio signals, base band digital signal samples representative of the detected spread spectrum radio signals are generated. For each one of the detected spread spectrum radio signals, the baseband signal samples are correlated with respect to the pilot spreading code to obtain correlated pilot spreading code signal samples. For each one of the detected spread spectrum radio signals, a frequency domain representation of a predetermined temporal window of the correlated pilot spreading code signal samples is formed. At least one covariance matrix using each frequency domain representation of the predetermined temporal window of the correlated pilot spreading code signal samples is formed. For each one of the detected spread spectrum radio signals, the baseband signal samples are correlated with respect to the data spreading code to obtain correlated data spreading code signal samples. For each one of the detected spread spectrum radio signals, a frequency domain representation of the correlated data spreading code signal samples is formed. The data is detected using each frequency domain representation of the correlated data spreading code signal samples in combination with the at least one covariance matrix.
In accordance with an added mode of the invention, for each of the steps of forming a frequency domain representation of a predetermined temporal window of the correlated pilot spreading code signal samples, the predetermined temporal window is taken in temporal correspondence with the predetermined sequence of pilot data. The at least one covariance matrix includes a signal plus interference and noise space-frequency covariance matrix. The signal plus interference and noise space-frequency covariance matrix is formed from each frequency domain representation of the predetermined temporal window of the correlated pilot spreading code signal samples.
In accordance with an additional mode of the invention, for each of the steps of forming a frequency domain representation of a predetermined temporal window of-the correlated pilot spreading code signal samples, the predetermined temporal window is taken at different temporal displacements with respect to a position of the predetermined sequence of pilot data. The at least one covariance matrix includes an interference and noise space-frequency covariance matrix. The interference and noise space-frequency covariance matrix is formed from each frequency domain representation of the predetermined temporal window of the correlated pilot spreading code signal samples.
In accordance with an another mode of the invention, for each of the steps of forming a frequency domain representation of a predetermined temporal window of the correlated pilot spreading code signal samples, the predetermined temporal window is taken at different temporal displacements with respect to a position of the predetermined sequence of pilot data. The at least one covariance matrix includes an interference and noise space-frequency covariance matrix. The interference and noise space-frequency covariance matrix is formed from each frequency domain representation of the predetermined temporal window of the correlated pilot spreading code signal samples.
In accordance with a further mode of the invention, the frequency domain representation of the correlated data spreading code signal samples is combined with the correlated pilot spreading code data signal samples to form a data signal plus interference and noise space-frequency snapshot.
In other words, the frequency domain representation of the correlated data symbols from each antenna are combined to form a data signal plus interference and noise space-frequency snapshot.
In accordance with a further added mode of the invention, a weight vector is estimated from a matrix selected from the group consisting of a signal space-frequency covariance matrix and a signal plus interference and noise space-frequency covariance matrix, in combination with an interference and noise-covariance matrix. A Hermittian transpose of the weight vector is multiplied by the data signal plus interference and noise space-frequency snapshot. The step of detecting the data is performed using the result of the multiplication.
In accordance with a further additional mode of the invention, the predetermined temporal window is taken substantially in accordance with a multiple path delay spread experienced by the radio signals.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method of communicating data using radio signals that includes generating spread spectrum radio signals representative of a combination of data and a data spreading code associated with a user, and representative of a combination of a predetermined sequence of pilot data and a pilot spreading code. The spread spectrum radio signals are detected at each one of a plurality of antennas. For each one of the detected spread spectrum radio signals, base band digital signal samples representative of the detected spread spectrum radio signals are generated. For each one of the detected spread spectrum radio signals, the baseband signal samples are correlated with respect to the pilot spreading code to obtain correlated pilot spreading code signal samples. For each one of the detected spread spectrum radio signals, a frequency domain representation of a predetermined temporal window of the correlated pilot spreading code signal samples is formed. At least one covariance matrix is formed using each frequency domain representation of the predetermined temporal window of the correlated pilot spreading code signal samples. For each one of the detected spread spectrum radio signals, the baseband signal samples are correlated with respect to the data spreading code to obtain correlated data spreading code signal samples. For each one of the detected spread spectrum radio signals, a frequency domain representation of the correlated data spreading code signal samples is formed. The data is detected using each frequency domain representation of the correlated data spreading code signal samples in combination with the at least one covariance matrix.
In accordance with a concomitant feature of the invention, the receiver operates to effect a two dimensional space-frequency rake receiver, in which space-frequency covariance matrices formed as part of the detection process are established using known data sequences communicated with the data. By exploiting the known pilot data sequences communicated with the user data to estimate the space-frequency covariance matrices, detection and recovery of the data from the spread spectrum radio signals is facilitated. Because radio signals from some users will be received with a considerably higher signal strength than radio signals received from other users, data symbols transmitted by users with strong signals will be more easily detected than symbols from users with relatively weaker radio signals. This is known to those skilled in the art as the near-far problem. The advantages of using a single user symbol detector based on adaptive antennas combined with multi-user interference cancellation in the space frequency domain, as effected by the receiver according to the present invention, is that a reduction in the near-far effect is facilitated.
Furthermore, compared to a two-dimensional space time rake receiver, the two-dimensional space frequency rake receiver is not limited by or to a finite number of rake fingers. As a result of the reduction in the near-far effect, a radio communication system operating with the receiver can be effected with substantially less stringent power control requirements. Furthermore, the receiver is able to detect the data with a substantial reduction in the complexity as compared to a space time rake receiver.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied as a receiver and a method of recovering data from radio signals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.